I have been having issues with large Vec (based on DMPLex) and massive MPI I/O  ... it looks like the data that is written by the Petsc Binary Viewer is gibberish for large meshes split on a high number of processes. For instance, I am using a mesh that has around 50 million cells, split on 1024 processors.

The computation seems to run fine, the timestep computed from the data makes sense so I think internally everything is fine. But when I look at the solution (one example attached) it's noise - at this point it should show a bow shock developing on the left near the step.

The piece of code I use is below for the output :

                call DMGetOutputSequenceNumber(dm, save_seqnum, save_seqval, ierr); CHKERRA(ierr)
                call DMSetOutputSequenceNumber(dm, -1, 0.d0, ierr); CHKERRA(ierr)
                write(filename,'(A,I8.8,A)') "restart_", stepnum, ".bin"
                call PetscViewerCreate(PETSC_COMM_WORLD, binViewer, ierr); CHKERRA(ierr)
                call PetscViewerSetType(binViewer, PETSCVIEWERBINARY, ierr); CHKERRA(ierr)
                call PetscViewerFileSetMode(binViewer, FILE_MODE_WRITE, ierr); CHKERRA(ierr);
                call PetscViewerBinarySetUseMPIIO(binViewer, PETSC_TRUE, ierr); CHKERRA(ierr);
                call PetscViewerFileSetName(binViewer, trim(filename), ierr); CHKERRA(ierr)
                call VecView(X, binViewer, ierr); CHKERRA(ierr)
                call PetscViewerDestroy(binViewer, ierr); CHKERRA(ierr)
                call DMSetOutputSequenceNumber(dm, save_seqnum, save_seqval, ierr); CHKERRA(ierr)

I do not think there is anything wrong with it but of course I would be happy to hear your feedback.

Nonetheless my question was : how far have you tested the binary mpi i/o of a Vec ? Does it make some sense that for a 50 million cell mesh split on 1024 processes, it could somehow fail ?

Or is it my python drawing method that is completely incapable of handling this dataset ? (paraview displays the same thing though so I'm not sure ...)

Thank you very much for your advice and help !!!